Electron beam and deflection yoke alignment for producing convergence of plural in-line beams

ABSTRACT

In a color television display system in which a plurality of horizontal in-line electron beams are directed toward color phosphor elements of a color picture tube, substantial convergence of the beams at all points on a scanned raster is achieved by transversely positioning the deflection yoke relative to the picture tube while maintaining substantial parallelism between the central longitudinal axes of the yoke and picture tube.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for aligning aplurality of electron beams with the magnetic field of a deflection yokefor producing convergence of the beams on a scanned raster.

The most common type of color television receiver in current useutilizes a delta gun shadow mask picture tube in which three electronbeams emanating from the apices of a triangle formed by the guns aredeflected to scan a raster on the phosphor screen. It is essential thatthe electron beams remain converged at all points on the screen so thatthe three different color rasters are superimposed. To this end it iscommon practice to utilize an electromagnetic dynamic convergencecorrection apparatus disposed around the neck of the color picture tubein the region where the beams leave the gun. This convergence correctionapparatus usually comprises three electromagnets disposed around threeinternal pole pieces of the picture tube, which electromagnets areenergized at the line and field scanning rates for dynamicallycontrolling the amount of correction imparted to the respective beams toensure satisfactory convergence at all points on the screen. Thewaveforms applied to the electromagnets may be suitably adjusted forproviding the desired correction. Although this dynamic correctionapparatus performs satisfactorily, it is relatively complex and hencerelatively expensive, thereby adding to the cost of the color televisionreceiver. Furthermore, the complexity of the apparatus requiresconsiderable time to be adjusted correctly.

Color television picture tubes utilizing three horizontal in-lineelectron beams have been used in some instances to replace the delta gunpicture tubes to simplify the apparatus required to maintain convergenceof the beams. For example, in a color television picture tube utilizingthree horizontal in-line beams and a phosphor element viewing screenutilizing a pattern of vertical different color phosphor strips,apparatus providing dynamic convergence can be simplified, althoughdynamic convergence is still required. It is obviously desirable toprovide a color television receiver which requires no dynamicconvergence correction at all.

Accordingly, it is an object of this invention to provide a method ofelectron beam and deflection yoke alignment for producing convergence ofplural in-line beams.

It is another object of this invention to provide apparatus for electronbeam and deflection yoke alignment for producing convergence of pluralin-line beams.

In accordance with the invention a method is provided of mounting a beamdeflection yoke relative to a color picture tube including an electrongun for projecting a plurality of in-line electron beams toward aphosphor screen, comprising supporting the picture tube in apredetermined position, temporarily supporting the yoke loosely aroundthe picture tube, operating the tube and yoke to project the beamstoward the screen and to scan separate rasters thereon andsimultaneously adjusting the transverse position of a yoke relative tothe picture tube while maintaining substantial parallelism of thelongitudinal axis of the yoke and the picture tube to achievesubstantial convergence of the beams on the screen, and fixedlyattaching the yoke to the picture tube in the adjusted position.

A system for converging a plurality of beams in a color televisiondisplay system includes a color picture tube including a color phosphorelement screen at one end thereof and an electron gun assembly within aneck portion of the tube at the other end thereof for producing aplurality of in-line beams. A deflection yoke adapted to be energized tocause the beams to scan rasters on the phosphor element screen has asmallest inner diameter larger than the outer diameter of the neckportion of the picture tube. Means are provided for allowing thedeflection yoke to be adjusted in directions transvere to the centrallongitudinal axis of the picture tube while maintaining substantialparallelism between the central longitudinal axis of the yoke and thepicture tube and for fixedly retaining the yoke in a position providingsubstantial convergence of the beams on the phosphor element screen.

The invention will be described more fully in the following descriptionof a preferred embodiment of the invention and in the accompanyingdrawings, of which:

FIG. 1 illustrates a system according to the invention for producingconvergence of a plurality of in-line beams; .Iadd.FIG. 1 is a partialsectional view, taken horizontally along the central beam axis of thepicture tube shown in FIG. 1; .Iaddend.

FIG. 2 illustrates a characteristic of a magnetic deflection fieldproduced by the deflection yoke shown in FIG. 1;

FIGS. 3 and 4 illustrate the effects of the magnetic deflection field ofFIG. 2 on the two outside electron beams shown in FIG. 1; .[.and.].

FIG. 5 is a partial cross-sectional view of the deflection yoke andpicture tube shown in FIG. 1.[...]..Iadd., taken vertically transverseto the central beam axis of the picture tube; and

FIG. 6 illustrates an electron gun assembly suitable for use in thesystem of FIG. 1; FIG. 6 is a partial sectional view, taken horizontallyalong the central beam axis of the picture tube of FIG. 1. .Iaddend.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a system according to the invention for producingconvergence of a plurality of in-line electron beams. FIG. 1 representsthe main display components of a color television receiver. A colortelevision picture tube comprises a glass envelope 11. At one end ofenvelope 11 is a transparent face plate 12 on the inside of which arerepeating groups of blue, red and green phosphor elements 13a, 13b and13c, respectively. Disposed a short distance from the phosphor elementsis an aperture mask 14 containing therein a plurality of apertures 15.Disposed within a neck portion of envelope 11 at the other end of thepicture tube is an electron gun assembly 16 for producing threehorizontal in-line beams 17a, 17b and 17c. The beams are modulated inaccordance with video signals to be respectively representative of blue,red and green color information of the televised scene. Disposed aroundthe neck portion of the envelope of the picture tube and adjacent theflared portion of the envelope 11 is a deflection yoke 18 which, whensuitably energized by sources of vertical and horizontal rate scanningcurrents, causes the electron beams 17a, 17b and 17c to scan respectiverasters over the phosphor elements. Deflection yoke 18 is held in placeby yoke mounting means 18a to be described in more detail subsequently.

Disposed around the neck region of the picture tube in the region wherethe electron beams leave electron gun assembly 16 are a static beamconvergence assembly 19 and beam purity device 20. The static beamconvergence assembly 19 may include a plurality of permanent magnets forstatically converging the beams at the center of the phosphor screen.Beam purity device 20 comprises two annular magnetized rings which aresimultaneously adjusted for achieving purity, i.e., to insure that therespective color representative beams land on their respective colorphosphor elements.

In a preferred embodiment the phosphor elements 13a, 13b and 13c arevertical strips of phosphor material, and the apertures 15 are elongatedslits also extending in a vertical direction. Such a vertical linephosphor element structure eliminates vertical registration problemsbecause even with a slight misalignment of the beams in a verticaldirection, a particular beam will still land on its intended colorphosphor because the phosphor strips extend throughout the verticaldimension of the viewing face plate. However, vertical misalignment ofthe beams will cause a problem in convergence which, together with itssolution, will be described subsequently. The arrangement thus fardescribed in FIG. 1 is disclosed in detail in copending U.S. applicationSer. No. 217,780, filed Jan. 14, 1972.[.,.]. for A. M. Morrell et al.and entitled "SELF-CONVERGING COLOR TELEVISION DISPLAY SYSTEM".Iadd.,which was refiled June 29, 1973 as continuation application Ser. No.374,831, now U.S. Pat. No. 3,892,996. In particular, the Morrell et al.application Ser. No. 217,780 discloses an electron gun assembly 16,illustrated in FIG. 6, that is suitable for use in the system of FIG. 1.Three separate cathodes 35a, 35b and 35c are provided for generatingthree electron beams. The electrons emitted by the cathodes aresubsequently accelerated, formed into beams and focussed by theremaining electrodes including a G1 electrode 36, a G2 electrode 37, aG3 electrode 38 and a G4 electrode 39. Although not shown, it is to beunderstood that the cathodes and the other electrodes are retained intheir relative positions by common suitable glass beading stripsattached to the various electrodes. Electron gun assembly 16 providesthe three electron beams 17a, 17b and 17c which converge at the centerof the faceplate 12 of FIG. 1 in the absence of a deflection fieldprovided by the deflection yoke 18. To achieve this converged conditionthe alignment and spacing of the various electrodes, particularly G3 andG4, relative to each other are critical. It should be noted that all ofthe electrodes have three apertures and are common to the three beams17a, 17b and 17c. This monolithic type of construction greatlyfacilitates the building of a precision electron gun which produces thedesired alignment of the beams particularly in the vertical direction.The spacing of the apertures in the G3 and G4 electrodes relative toeach other enables the two outside beams 17a and 17c to converge on thecenter beam 17b at the viewing screen. Minor errors in beam alignment(convergence at the center of the viewing screen) are corrected bysuitable adjustment of the static convergence assembly 19 referred toabove. A suitable electron gun assembly of the type described isdisclosed in more detail in copending application Ser. No. 217,758 filedconcurrently herewith for Richard Hughes and entitled "In-Line ElectronGun For Color Picture Tube", now U.S. Pat. No. 3,772,554.Iaddend..

FIG. 2 illustrates a characteristic of a magnetic deflection fieldproduced by the deflection yoke shown in FIG. 1. As described in theaforementioned Morrell et al. application .Iadd.Ser. No.217,780.Iaddend., the particular selection of the electron gun assembly16 and the deflection yoke 18 will provide substantial convergence ofthe beams at all points on the scanned raster without requiring the useof dynamic convergence correction apparatus. The characteristics of adeflection yoke for achieving this are that it exhibit negativehorizontal isotropic astigmatism and positive vertical isotropicastigmatism. FIG. 2 illustrates the net or dominant magnetic fieldproduced by a yoke exhibiting these astigmatic characteristics. In FIG.2 lines of flux 25 illustrate the horizontal magnetic field. Thepincushion shape of the field is due to the negative horizontalisotropic astigmatism. This pincushion-shaped deflection field increasesin strength with distance from the center along an axis parallel to thehorizontal direction of deflection. At the same time thispincushion-shaped field decreases in strength along an axisperpendicular to the horizontal direction of the deflection.

In FIG. 2 lines of flux 26 illustrate the vertical magnetic deflectionfield produced by a deflection yoke exhibiting positive verticalisotropic astigmatism. This vertical field is barrel-shaped and thevertical barrel-shaped deflection field gets stronger along thehorizontal axis and weaker along the vertical axis. A preferredembodiment of a deflection yoke exhibiting this type of field isdisclosed in copending U.S. application Ser. No. 217,768, filed Jan. 14,1972, for W. H. Barkow et al. and entitled "DEFLECTION YOKE FOR USE WITHIN-LINE ELECTRON GUNS."

The deflecting yoke magnetic field distribution described in conjunctionwith FIG. 2 is designed to converge perfectly aligned electron beams. Ifthe beams are not properly aligned with the center of the yoke magneticfield, the beams will not be converged on the viewing screen.Misalignment of the beams relative to the yoke magnetic field couldoccur with electron gun misalignment within the picture tube, deflectionyoke misalignment relative to the beams or the tube neck, or nonsymmetryof the deflection yoke coils. It can be generally stated thatmanufacturing variations in the electron gun assembly displacementwithin the picture tube or in the winding of the deflection yoke willresult in misalignment of the beams relative to the yoke magnetic field.In a system such as described in FIG. 1 in which no dynamic convergencecorrection apparatus is utilized, the beams will not be converged unlesssteps are taken to align the beams with the magnetic field in accordancewith the invention.

FIGS. 3 and 4 illustrate the effects of the magnetic deflection field ofFIG. 2 on the two outside ones of the three in-line electron beams whenthe beams are misaligned relative to the deflection yoke magnetic field.FIG. 3a illustrates the condition in which the blue and green beams 17aand 17c are misaligned in a vertical direction relative to the center ofthe horizontal deflection field illustrated by the lines of flux 25. Thedotted lines indicate the magnetic flux and the solid arrows that pointaway from the beams indicate the general direction and amount of beamdisplacement caused by the misaligned position of the beams. To simplifythe illustrations in FIGS. 3 and 4, the center one of the three beams,red beam 17b, has been omitted because it can generally be stated thatthe effect on the center beam will be to cause it to lie between theoutside blue and green beams.

In FIG. 3a the blue and green beams see the same strength magnetic fieldbecause they are equally displaced in a horizontal direction from thecenter of the field, but the directions are different such that, whenthe beams are deflected to the right, the blue beam is also deflectedupward and the green beam is deflected downward. When deflected to theleft (by reversing the indicated polarity of the magnetic field), theblue beam would be undesirably deflected downward and the green beamupward. The result is that along the horizontal or X-axis the blue beamwill be low on the left side of the raster and high on the right withrespect to the green beam. If the beams had been misaligned downwardfrom the center axis rather than upward, the blue beam would be high onthe left and low on the right relative to the green beam.

In FIG. 3b the blue and green beams are misaligned horizontally to theright in the horizontal deflection field 25. As described above, thepincushion-shaped horizontal deflection field increases in strength withdistance from the center along the horizontal axis. Therefore, themisaligned green beam being farther from the center than the blue beamis in a stronger magnetic field and is deflected further. The result isthat the green raster is wider than the blue raster. It can be seen thatif the polarity of the deflection field were reversed such as when thebeams are to be deflected to the right side of the raster, the greenbeam would be similarly deflected further than the blue. Had the beamsbeen misaligned to the left of the center rather than to the right asillustrated, the green raster would be smaller than the blue raster.

FIG. 4a illustrates the effect on the beams when they are misalignedvertically, in upward direction, under the influence of the verticalmagnetic deflection field illustrated by the lines of flux 26. The bluebeam will move to the left at the top of the raster and to the right atthe bottom of the raster with respect to the green beam due to thedirection of the magnetic flux lines.

FIG. 4b illustrates the effect of a horizontal misalignment of the beamsin the vertical deflection field. Because the green beam is farther fromthe center axis than is the blue, it is in a stronger portion of thefield and hence will be deflected a greater distance vertically than theblue beam. This causes the green raster to be larger in a verticaldirection both top and bottom than the blue raster.

It has been shown how misalignment of the electron beams relative to thevertical and horizontal magnetic field causes misconvergence. Horizontalmisalignment of the beams causes the rasters to vary in size bothvertically and horizontally. Vertical misalignment of the beams causesthe rasters formed by the two outside beams to be rotated in oppositedirections from each other.

FIG. 5 is a partial cross-sectional view of the deflection and picturetube shown in FIG. 1. In accordance with the invention it has beendetermined that misconvergence of the beams caused by misalignment ofthe beams relative to the magnetic field of the deflection yoke can begreatly reduced by positioning the deflection yoke relative to the beamsso as to bring the magnetic deflection center of the yoke in alignmentwith a point halfway between the two outside beams. In FIG. 5 adeflection yoke 18, in this case having conductors 21 only a portion ofwhich are illustrated, toroidally wound about a core is designed suchthat the smallest inner diameter of the yoke is larger than the outsidediameter of the neck portion of the picture tube 11. With such anarrangement the deflection yoke may be shifted laterally in an X and Ydirection or in any direction in the X-Y plane so as to cause themagnetic field of the yoke to be aligned with the electron beams toproduce substantial convergence of the beams on the phosphor screen. Itis important to note that in accordance with a feature of thisinvention, a deflection yoke in being positioned in the X-Y plane aboutthe picture tube envelope 11 is moved transversely such that the centrallongitudinal axis of the yoke or central longitudinal axis of the yokedeflection field is aligned with the central beam axis or is parallelthereto. In the past yoke mounts have been provided which permit theyoke to be tilted with respect to the neck of the picture tube such thatthe yoke would no longer lie in an X-Y plane. This tilting serves tocorrect for coma in delta gun color television picture tubes whichresulted in the blue raster being of different width than the red andgreen rasters. With such an arrangement dynamic convergence correctionof all of the beams was required to properly converge the beams at thephosphor screen. It has been determined that in the arrangementdescribed in FIG. 1, mere tilting of the deflection yoke could result inmisregistry of the beams on the phosphor screen. However, by providingthe described transverse movement of the entire yoke, the registrationof the beams on the color phosphor elements is not disturbed andconvergence is achieved without requiring dynamic convergence correctionapparatus.

It has been determined that for a color television picture tube such asdescribed in conjunction with FIG. 1 having a viewing screen diagonal of15 inches, a radial clearance of about 50 mils between the outerdiameter of the picture tube neck and the smallest inner diameter of thedeflection yoke provides enough clearance for moving the yoke forproducing substantial convergence of the beams. It has been determinedthat for a shift of about 50 mils in a horizontal direction of the yokeresulted in a 50 mil change in the convergence at the horizontal sidesof the rasters. Thus, in accordance with the illustrations in FIGS. 3and 4, a horizontal yoke shift of about 50 mils to the right produces a50 mil increase in size of the blue raster at each side with respect tothe green, as the blue raster size is increased and the green rastersize is decreased. For this same horizontal shift the blue rasterbecomes about 25 mils bigger than the green at the top and bottom of theraster. The sensitivity to vertical shift of the yoke is approximatelythe same as it is for the described horizontal shift. The changes aresuch that the blue and green rasters are moved in opposite directionswith respect to the red, which itself lies between the blue and green.The convergence changes in the corners as the yoke is moved is acombination of the convergence changes on the adjacent vertical andhorizontal axes.

It can be seen that a relatively small shift of the yoke can providesubstantial convergence correction. Therefore, it is very desirable thatthe yoke be rigidly retained in the desired operating position relativeto the picture tube. To this end suitable yoke mounting means areprovided which allow the yoke to be properly positioned for the bestoverall convergence condition and then rigidly fixed in position.Several convenient ways of mounting a yoke in the manner described aredisclosed in copending U.S. application Ser. No. 217,756, filed Jan. 14,1972, for T. M. Shrader and entitled "CATHODE RAY TUBE-YOKEPLATFORM-YOKE COMBINATION AND METHOD OF ASSEMBLING THE COMBINATION." Inthat application one arrangement for mounting the yoke is to permanentlybond a mounting platform on the picture tube, permanently attaching amounting member to the deflection yoke and loosely assembling the yokemounting member to the platform while the yoke and tube are operated todetermine the optimum position for the yoke on the picture tube. Whenthis position is ascertained such as by observing the convergence of thelines of a cross-hatched pattern caused to appear on the picture tube byapplication of a suitable test signal applied to the televisionreceiver, the yoke mounting member and the platform are rigidly fixed toeach other by means of a bonding agent or by mechanical means such asnuts and bolts holding the two pieces together.

The invention is not limited by the particular yoke mounting meansprovided. It should be noted that any mounting means allowing transversemovement of the entire yoke rather than tilting movement alone andpermitting the yoke to be fixedly retained in a desired position issuitable for use with the invention.

The method of aligning the yoke magnetic field with the beams to achieveconvergence and the apparatus for holding the yoke permanently in thedesired operating position would be utilized during manufacture of thetelevision receiver as the yoke and tube are operated for producing asuitable pattern such as a cross-hatch pattern on the phosphor screen sothe operator can observe and determine the optimum placement of the yokefor best convergence.

The invention has been described in the context of a color televisiondisplay system in which the features of the invention eliminate the needfor dynamic convergence correction waveforms and apparatus. However, theinvention may also be well utilized with dynamic convergence correctionapparatus to supplement the degree of convergence correction of thesystem or to simplify and reduce the cost of the existing convergencecorrection system such as by eliminating some or all of the variablecontrol elements or by reducing the number of convergence correctionwaveform generating circuit components.

What is claimed is:
 1. A method of mounting a beam deflection yokerelative to a color picture tube comprising an envelope containing acolor phosphor screen, an electron gun assembly for projecting aplurality of electron beams toward said screen and a deflection regionfor said beams, said method comprising:supporting said tube in apredetermined position; temporarily supporting said deflection yokeloosely assembled around said picture tube adjacent said deflectionregion; operating said tube and yoke to project said beams toward saidscreen and to scan said beams in separate rasters on said screen, andsimultaneously adjusting the transverse position of said yoke relativeto said picture tube while maintaining .[.substantial.]. parallelism ofthe longitudinal axes of said yoke and picture tube to achievesubstantial convergence of said beams and coincidence of said rasters onsaid screen .Iadd.without the use of dynamic convergence correctionmeans.Iaddend.; and fixedly attaching said yoke to said picture tube insaid adjusted position.
 2. A method according to claim 1 wherein saidelectron gun assembly is selected for producing three coplanar in-linebeams.
 3. A method according to claim 2 wherein said yoke comprises apair of vertical and a pair of horizontal deflection coils toroidallywound around a core, the conductor winding distribution of said coilsbeing selected for producing positive vertical isotropic astigmatism andnegative horizontal isotropic astigmatism.
 4. A method according toclaim 3 wherein said phosphor screen comprises strips of different colorphosphor elements.
 5. In a color image display system, the combinationcomprising:a color picture tube including an envelope containing ascreen of different color phosphor elements and an electron gun assemblydisposed within a neck portion of said tube envelope for producing aplurality of beams for impinging on said respective color phosphorelements; a deflection yoke adapted to be energized to cause said beamsto scan respective rasters on said phosphor screen, said yoke having asmallest inner diameter larger than the outer diameter of said neckportion of said tube envelope; and means for mounting said yoke on saidpicture tube, said means being selected for allowing transverse movementof said yoke relative to said picture tube while maintaining.[.substantial.]. parallelism of the central longitudinal axes of saidyoke and said picture tube such that said yoke can be fixedly mounted ina position providing substantial convergence of said beams andcoincidence of said rasters .Iadd.without the use of dynamic convergencecorrection means.Iaddend..
 6. A system for converging a plurality ofbeams in a color television display system, comprising:a color picturetube comprising an envelope containing a color phosphor element screenat one end thereof and an electron gun assembly for producing threehorizontal coplanar in-line beams disposed within a neck portion of saidenvelope remote from said screen; .Iadd.said electron gun assemblyincluding, near that end of the electron gun assembly from which theelectron beam exit, at least two adjacent common electrodes, each commonelectrode having, for each electron beam, a separate aperture, throughwhich the beam passes, so that a selected alignment of the beamsrelative to one another can be produced; .Iaddend. a deflection yokehaving the conductor winding distribution of horizontal and verticaldeflection coils thereof selected for producing negative horizontalisotropic astigmatism and positive vertical isotropic astigamatism, saidcoils being spaced circumferentially around a core and forming asmallest inside diameter of said yoke larger than the outside diameterof said neck portion of said tube envelope; and means for mounting saidyoke around said picture tube and including means for allowing movementof said entire yoke in directions substantially normal to the centrallongitudinal axis of said picture tube and for fixedly retaining saidyoke in a position providing substantial convergence of said beams.Iadd.without the use of dynamic convergence correction means. .Iaddend.7. A system for converging a plurality of beams according to claim 6wherein said deflection yoke produces underconvergence of said beamsalong the horizontal axis of deflection and overconvergence of saidbeams along the vertical axis of deflection.
 8. A system for converginga plurality of beams according to claim 7 wherein said phosphor screenincludes a repeating pattern of red, green and blue vertically extendingphosphor strips. .Iadd.
 9. A system for converging a plurality of beamsin a color television display system, comprising:a color picture tubecomprising an envelope containing a color phosphor element screen at oneend thereof and an electron gun assembly for producing three horizontalcoplanar in-line beams disposed within a neck portion of said enveloperemote from said screen; said electron gun assembly including, near thatend of the electron gun assembly from which the electron beams exit, atleast two adjacent common electrodes, each common electrode having, foreach electron beam, a separate aperture, through which the beam passes,the spacing of the apertures of said two adjacent electrodes relative toeach other producing a selected alignment of the beams relative to oneanother; a deflection yoke having the conductor winding distribution ofhorizontal and vertical deflection coils thereof selected for producingnegative horizontal isotropic astigmatism and positive verticalisotropic astigmatism, said coils being spaced circumferentially arounda core and forming a smallest inside diameter of said yoke larger thanthe outside diameter of said neck portion of said tube envelope; andmeans for mounting said yoke around said picture tube and includingmeans for allowing movement of said entire yoke in directionssubstantially normal to the central longitudinal axis of said picturetube and for fixedly retaining said yoke in a position providingsubstantial convergence of said beams without the use of dynamicconvergence correction means. .Iaddend.